Where Have The Gigantic Animals Gone?

The era of the Mesozoic was about 225 - 65 million years ago.
The term Mesozoic derives from the ancient Greek μέσος (mésos) which means "intermediate" and ζωή (zoḗ) meaning "life", to indicate the period of intermediate fauna between the Paleozoic and the Cenozoic.
Cretaceous, from 145.5 Ma to 65.5 Ma (Millions of years ago)
Jurassic, from 199.6 Ma to 145.5 Ma
Triassic, from 251 Ma to 199.6 Ma
It could also be that Dinosaurs no longer exist for a climate condition.
This video explains a bit about this.